There have been widespread combline-type microstripline filters in which open ends of striplines constituting quarter-wavelength resonators are unidirectionally placed such that adjacent resonators couple to each other. In the combline-type microstripline filter, each resonant line may have a step structure in which the line width of an open-end-side electrode is different from the line width of a short-circuit-end-side electrode. Also, in the microstripline filter having the step structure, a depressed portion may be provided in an open-end-side electrode so that a short-circuit-end-side electrode extends from the bottom of the depressed portion of the open-end-side electrode to a ground electrode (e.g., see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-111602